The present disclosure relates to digital printing and particularly to printing processes in which the physical image-forming material such as, for example, dry powder toner in a xerographic marking engine is rendered into at least a portion of the desired final image on some internal member such as, for example, a photoreceptor or passive receiver element and subsequently transferred to the final substrate. In such printing it has been found difficult to maintain the quality of the printed image, particularly for color prints where slight variation in color or color uniformity can render the print unacceptable to the customer.
A well-known technique for controlling print quality is the rendering of test images of the physical image-forming material and their measurement within the machine. Such measurements are then compared to some desired reference value and any difference used to determine adjustments to the printing process. Such test images may be of greater or lesser extent and may be measured in various ways, as, for example, for average value, or for uniformity over an extended area. The rendering of test images may require the interruption of the printing of the desired images, or may be arranged to take place while printing proceeds, as, for example, by rendering test images between the desired images. The measurement process itself may be affected by variation in the characteristics of the internal member or the sensor used for such measurement.
Such internal measurements may not, however, be capable of monitoring all aspects of print quality for which control is necessary or desired, for example defects due to variation in the transfer of images to the final substrate. A well-known technique for detecting and correcting for such defects is the measurement of test prints. Such measurement can take place using a sensor or sensors within the marking engine disposed so this can take place after the final step in the printing process, such as fusing, or by using an external scanner or other sensor. The production of such test prints requires the interruption of useful printing and therefore results in a reduction in marking engine productivity and consumes substrate material thereby increasing the cost of such measurements.
It has thus been desired to minimize the disadvantages of these two approaches, so as to provide better control of print quality and to reduce the cost, both in substrate material and lost productivity, of such control.